1. Technical Field
The present invention relates to multi-thread processing in general, and more particularly, to a method and apparatus for adjusting the sleep time of fixed high-priority threads.
2. Description of Related Art
The UNIX® operating system enables multiple threads to run concurrently in a user space. By default, a thread executing under a user mode typically has a low priority and can be preempted or interrupted at any time by other same or higher priority threads. Thus, in order to implement a real-time application in the user mode, some threads of the real-time application are assigned a high priority on a permanent basis, and those threads are commonly known as fixed high-priority threads.
The liberal usage of fixed high-priority threads can potentially cause low-priority threads to receive only a very small portion of processor time. In order to avoid the continued starvation of low-priority threads, certain UNIX® kernels utilize a sleep function, which implements a series of time-out windows, to put some fixed high-priority threads to sleep on a per request basis. After making a sleep request, a fixed high-priority thread is put to sleep by the sleep function, and will be awoken at the beginning boundary of a time-out window in which the sleep request ends. Low-priority threads can be executed when the fixed high-priority thread is sleeping.
The size of each time-out window implemented by the sleep function is constant, and each time-out window occurs on a periodic schedule. The duration of each sleep request can only be made in multiples of the size of a time-out window. But regardless of its requested sleep duration, a fixed high-priority thread will be awoken at the beginning boundary of a time-out window in which a sleep request ends. In other words, no matter when the wake up time is to occur within a time-out window, the fixed high-priority thread will be awoken at the beginning boundary of the same time-out window. For example, if the size of each time-out window is 10 ms, and a sleep request for a 10 ms duration is made by a fixed high-priority thread at the 7 ms mark of a first time-out window, then the fixed high-priority thread will be awoken at the beginning boundary of the second time-out window. The result is a sleep time of 22 ms instead of the requested 30 ms. As another example, if the size of each time-out window is 10 ms, and a sleep request for a 30 ms duration is made by a fixed high-priority thread at the 8 ms mark of a first time-out window, then the fixed high-priority thread will be awoken at the beginning boundary of the fourth time-out window. The result is a sleep time of 22 ms instead of the requested 30 ms. Thus, when a fixed high-priority thread repeatedly makes sleep requests near the end boundary of a time-out window, the fixed high-priority thread will not receive adequate sleep time such that low-priority threads will be deprived of their execution time.
One solution for the above-mentioned problem is to reduce the size of time-out windows, but it may result in an overall decrease in system performance due to an increase in timer interrupts. Consequently, it would be desirable to provide an improved method for adjusting the sleep time of fixed high-priority threads.